The present invention is directed to automatic circuit-board testing. It particularly concerns digital in-circuit testing.
There is usually a stage in printed-circuit-board manufacture at which the boards are subjected to so-called in-circuit tests. This type of test is performed before any overall functional testing, i.e., before any testing of the board's overall functional characteristics, with which in-circuit testing does not concern itself. In-circuit testing instead focuses on the performance of individual components after they have been placed on the circuit board.
In an in-circuit test, probes contact the board primarily at internal board nodes, typically circuit-board traces, rather than at board terminals. Digital drivers apply signals to some of these traces, and digital sensors determine whether proper signal levels result at these or other similarly probed traces. The ensemble of the digital levels to be applied and/or sensed at all of the test nodes at a given clock time is known as a vector, and the tester includes fast pin memories that together contain codes for a large number of such vectors. A sequencer causes a burst of those codes to be fetched and the resultant levels to be applied to the drivers and sensors at a rapid rate, and the sensors' results are simultaneously loaded back into the pin memories to record the results of the test. Each component's test requires one or more such bursts, so a board containing many components requires many bursts to test it.
Between bursts, the circuit board's components are allowed to cool down and the pin memories are reloaded with new vectors. Tester multiplexers known as "scanners" may also be operated during the inter-burst interval to change the connections between board nodes and digital instruments. (There are typically are many fewer digital instruments than probed nodes, because only a subset of the probed nodes needs to be driven and/or sensed during any individual burst.)
If a circuit board and its components are very simple, then the time required for an in-circuit test may be so small that the most-significant factor affecting tester throughput is the time required to load the board into the tester and remove it again. For most of the more-complex circuit boards, though, the time required for the actual electrical testing can be quite long, so test length has a significant effect on the testing volume of which a given number of board testers are capable.
Unfortunately, the very nature of in-circuit testing tends to make it a slow process. This is a result of the need to isolate individual components during an in-circuit test. Specifically, the board is powered up during an in-circuit test, so components other than the one currently being tested may be attempting to drive a board trace to a level different from the one that the test specifies. Various techniques are employed to minimize other component's tendencies to interfere in this manner; it is often possible to apply signals that will disable those neighbor components, for instance. But not all components can be disabled. Circuit testers typically deal with such components by backdriving, i.e., by driving the node with enough current to overpower the conflicting component.
Although it is possible in principle to make backdrive-capable digital drivers very fast, doing so is usually too expensive to be practical, so backdrive-capable digital drivers ordinarily can operate only at speeds considerably slower than, say, the speeds at which the boards being tested will operate in their intended environment. So it typically is not practical to increase test-vector-application speed significantly. Moreover, the high backdrive currents heat board components up during a burst, so cool-down time must be allowed between bursts.